The present invention relates to a thermosensitive image transfer medium utilizing a thermal coloring reaction between a leuco dye and a color developer, which is capable of yielding colored images with high and uniform image density by application of a small quantity thermal energy thereto even if image transfer is done multiple times.
Conventionally, there are known several thermosensitive image transfer mediums. For example, a thermosensitive image transfer medium consisting of (i) an image transfer sheet comprising a sublimation-type dye layer formed on a support material and (ii) an acceptor sheet capable of accepting the sublimated dye images from the sublimation-type dye layer of the image transfer sheet when thermal printing is performed from the back side of the image transfer sheet.
Another conventional thermosensitive image transfer medium consists of (i) an image transfer sheet comprising an image transfer layer formed on a support material, which image transfer layer comprises a thermofusible material and a pigment or a dye, and (ii) an acceptor sheet.
The former thermosensitive image transfer material has the shortcomings that the dye images on the acceptor sheet are poor in preservability because of the use of the sublimation-type dye and therefore an overcoating must be provided on the transferred images.
In the latter thermosensitive image transfer medium, the image transfer layer contains a pigment or a dye dispersed in the thermofusible material. If a large quantity of the pigment is contained in the image transfer layer in an attempt of obtaining images with high density, the image transfer ratio decreases. The result is that it becomes difficult to obtain images with high density. If a large quantity of a thermofusible material is contained in the image transfer layer in order to increase the thermosensitivity, a large quantity of the thermofusible material is transferred from the transfer sheet to the acceptor sheet. As a result, it becomes difficult to peel the transfer sheet off the acceptor sheet smoothly, so that line images on the acceptor sheet become unclear.
In addition to the above-described conventional thermosensitive image transfer mediums, a further thermal printing type thermosensitive image transfer medium is known, in which materials which react with each other to form a color upon application of heat thereto are supported separately in the form of two layers, each layer on a different support material, and thermal printing is performed by bringing the two layers into close contact with each other. In thermosensitive image transfer mediums of this type, the coloring reaction does not occur sufficiently if the image transfer layer is merely transferred to the acceptor layer by bringing them into contact with each other, thus yielding images having low image density. If thermal printing were performed at high temperatures with application of heat for a long period of time for allowing the coloring reaction to take place sufficiently, images having high density would be obtained on the acceptor sheet. However, the coloring reaction would also take place on the image transfer sheet at the same time. In other words, image formation occurs on both the acceptor sheet and the image transfer sheet.
A further variety of thermosensitive image transfer mediums of the above-mentioned type have been proposed in an attempt to obtain transferred images with uniform density even if image transfer is performed multiple times from the same image transfer medium.
For instance, in a thermosensitive image transfer medium of the above type, a porous filler having an oil absorption of 50 ml/100 g or more is contained in an image transfer layer which comprises as the main component a leuco dye.
In another thermosensitive image transfer medium, the image transfer sheet comprises a plastic film, a porous-filler-containing layer which contains a porous filler having an oil absorption of 50 to 300 ml/100 g formed on the plastic film and an image transfer layer comprising as the main component a leuco dye formed on the porous-filler-containing layer.
In a further thermosensitive image transfer medium, the image transfer sheet comprises a support material having a 0.5 to 3 .mu.m surface roughness and an image transfer layer formed on the support material.
These conventional thermosensitive image transfer mediums, however, are not capable of obtaining sufficiently high and uniform image density for use in practice at multiple image transfer from the same image transfer medium.